Compiling Stencils in High Performance Fortran
Brickner, R. Gregg; Kennedy, Ken; Mellor-Crummey, John; Roth, Gerald H.
DateNovember 12, 1997
For many Fortran90 and HPF programs performing dense matrix computations, the main computational portion of the program belongs to a class of kernels known as stencils. Stencil computations are commonly used in solving partial differential equations, image processing, and geometric modeling. The efficient handling of such stencils is critical for achieving high performance on distributed-memory machines. Compiling stencils into efficient code is viewed as so important that some companies have built special-purpose compilers for handling them and others have added stencil-recognizers to existing compilers. In this paper we present a general compilation strategy for stencils written using Fortran90 array constructs. Our strategy is capable of optimizing single or multi-statement stencils and is applicable to stencils specified with shift intrinsics or with array-syntax all equally well. The strategy eliminates the need for pattern-recognition algorithms by orchestrating a set of optimizations that address the overhead of both intraprocessor and interprocessor data movement that results from the translation of Fortran90 array constructs. Our experimental results show that code produced by this strategy beats or matches the best code produced by the special-purpose compilers or pattern-recognition schemes that are known to us.In addition, our strategy produces highly optimized code in situations where the others fail, producing several orders of magnitude performance improvement, and thus provides a stencil compilation strategy that is more robust than its predecessors.